[Sip] Of-hook and On-hook @ sip?

["F.S.Salloum" <ssal@intracom.gr>]

Dear all,

One simple question is there a similar message
in SIP which works like on-hook and off-hook on 
the PSTN world (Q-931)?

I know that this can be implemented playing with
the state machines of the hard-sip-phone but we were
wondering if this has been investigated from you guys.

Thanks
F.S.Salloum
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